1. Field of the Invention
The invention relates to hydrodewaxing hydrocarbons over an unsulfided dewaxing catalyst treated with oxygenates. More particularly the invention relates to the catalyst and to producing dewaxed fuel and lubricant oil fractions, from waxy hydrocarbons synthesized by reacting H2 and CO produced from natural gas in the presence of a Fischer-Tropsch catalyst, by hydrodewaxing the waxy hydrocarbons over an unsulfided dewaxing catalyst that has been treated by contacting it with oxygenates.
2. Background of the Invention
Fuels and lubricants are made by incorporating various additives into base stocks, which typically comprise dewaxed hydrocarbon fractions derived from waxy hydrocarbons that boil in the desired fuel and lubricant oil ranges. Dewaxing reduces the pour and cloud points of the waxy hydrocarbons, to acceptable levels. The relatively pure waxy and paraffinic hydrocarbons synthesized by the Fischer-Tropsch process are an excellent feed for producing diesel fuel, jet fuel and premium lubricant oils with low sulfur, nitrogen and aromatics contents. The sulfur, nitrogen, and aromatics content of these waxy hydrocarbons is essentially nil and the raw hydrocarbons can therefore be passed to upgrading operations, without prior hydrogenation treatment. In a Fischer-Tropsch process, H2 and CO react in the presence of a hydrocarbon synthesis catalyst to form waxy hydrocarbons. Those waxy hydrocarbon fractions that are solid at ambient conditions are referred to as Fischer-Tropsch wax and typically include hydrocarbons boiling in both the fuels and lubricant oil ranges. However, they have cloud and pour points too high to be useful as fuels and lubricant oils and must therefore be further processed (e.g., dewaxed) to meet acceptably low levels of cloud and pour points. Solvent dewaxing cannot be used, because the yield of dewaxed hydrocarbons boiling in the distillate fuels range will be substantially reduced and the higher molecular weight (e.g., C16+) hydrocarbons comprising the lubricant oil fractions are typically solid at ambient temperature. Various processes have been disclosed for catalytically dewaxing waxy hydrocarbons. Many, such as those employing a ZSM-5 catalyst, dewax by hydrocracking the waxy hydrocarbons to products boiling below the fuel and lubricant oil ranges. Others include hydroprocessing for removal of heteroatoms, aromatics and other unsaturates. Illustrative, but nonlimiting examples of various catalytic dewaxing processes are disclosed in, for example, U.S. Pat. Nos. 6,179,994; 6,090,989; 6,080,301; 6,051,129; 5,689,031; 5,075,269 and EP 0 668 342 B1.
More recently, catalysts that dewax mostly by isomerization have been discovered (as disclosed in, for example, in U.S. Pat. No. 5,075,269) and these produce greater dewaxed product yield, due to less cracking. However, even the best of these catalysts have some cracking activity and concomitant dewaxed product loss. Catalyst having a high cracking activity are especially undesired for the dewaxing of hydrocarbons that do not contain high amounts of waxes, e.g., because they have been produced using a non-cobalt Fischer-Tropsch catalyst. Sulfiding a dewaxing catalyst may reduce its cracking activity, as is well known in the art, but sulfiding may contaminate both the dewaxed product and the hydrogen reaction gas passing through the dewaxing reactor. It would be an improvement to the art if an alternative catalyst treatment could be found that does not require sulfiding and that does still give good hydrodewaxing results even with waxy hydrocarbons that do not contain high amounts of waxes. Also, the catalyst should have a reduced cracking activity during hydrodewaxing the hydrocarbons produced by noncobalt Fischer-Tropsch hydrocarbons synthesis, and thereby increase its isomerization dewaxing activity and contaminant dewaxed product yield.